This invention relates to an array of light-emitting diodes (hereinafter referred to as an LED array), and to a process for fabricating such an array with improved uniformity.
LED arrays are used, for example, to illuminate photosensitive drums in electrophotographic printers. The high density of diodes in the array and the need to obtain uniform optical output from each of the diodes makes the fabrication of LED arrays more difficult than the fabrication of individual light-emitting diodes.
One conventional fabrication method deposits an insulating film of aluminum oxide (Al.sub.2 O.sub.3) on a wafer comprising an n-type semiconductor substrate; patterns the insulating film by photolithography to form an array of rectangular openings; diffuses a p-type impurity such as zinc through the openings to form an array of p-type diffusion regions in the n-type substrate, thereby creating pn junctions; then deposits an aluminum film on the entire wafer and patterns the aluminum to form a set of electrodes, one electrode making contact with each of the p-type diffusion regions. Each pn junction functions as a light-emitting diode (LED).
One problem encountered in this method is the occurrence of pinholes in the insulating film. A pinhole disposed below an electrode can short-circuit the electrode to the n-type substrate, so that current bypasses the corresponding light-emitting diode and no light is emitted from that diode. A pinhole occurring at the periphery of one of the openings in the insulating film can enlarge its light-emitting area; the underlying diode then differs from other diodes in the shape and power of its emitted light pattern. In a printer, pinholes result in missing or uneven dots.
A second problem concerns the patterning of the insulating film. The patterning process includes a wet etching step employing a liquid etchant, which is attracted by surface tension toward the corners of the rectangular pattern openings. Pooling of the etchant in the corners can lead to uneven etching, hence to uneven diffusion of the p-type impurity. If pooling causes over-etching of the insulating film between adjacent rectangular openings, for example, p-type diffusion bridges may form between adjacent diodes. Similar pooling can occur at corners of the patterns used to form the aluminum electrodes, leading to variability in the area occupied by different electrodes within the p-type diffusion regions. In a printer, these problems again result in uneven dots, or in fused dots.
A second conventional fabrication method seeks to solve these problems by employing a bi-layer insulating film, the lower layer comprising silicon nitride (Si.sub.3 N.sub.4) and the upper layer silicon oxynitride (SiO.sub.x N.sub.y). Both layers are etched by a dry etching step employing a gaseous etchant, thereby avoiding the problem of pooling of a liquid etchant. As for pinholes, although pinholes may form at random locations in each layer, the probability that pinholes will occur at the same location in both layers is in theory very small.
In practice, however, this second conventional method does not eliminate the problem of pinholes, because when the insulating film is etched, there may be pinholes in the photoresist etching mask. Etchant gas will gain access through these pinholes to the upper layer, etch through the upper layer to the lower layer, then etch through the lower layer to the substrate. After the photoresist is removed, pinholes extending through both the upper and lower layers of the insulating film will be left.
Another problem with the second conventional method is that silicon nitride and silicon oxynitride give rise to greater internal stress than does; aluminum oxide. Such internal stress can lead to cracking or flaking of the insulating film, resulting in a defective array.